Final answer:
The unknown organism lacking a membrane-bound nucleus is most likely a prokaryote, which includes bacteria and archaea, as this is a defining characteristic of prokaryotic cells. So, correct answer is (A).
Step-by-step explanation:
An unknown organism that is discovered to lack a membrane-bound nucleus in its cell is most likely a prokaryote. Prokaryotic organisms include bacteria and archaea, which are characterized by having cellular DNA located in a region called the nucleoid, not separated by a nuclear membrane.
Unlike eukaryotes, prokaryotes do not have other membrane-bound organelles within their cells. A eukaryote has a well-defined nucleus enclosed in a nuclear membrane and contains additional organelles such as mitochondria. Viruses, though not considered living organisms, consist of genetic material enclosed in a protein shell and do not have a cellular structure.
Therefore, the unknown organism cannot be a virus or a eukaryote, and it is not a plant since plants are eukaryotic organisms with membrane-bound nuclei.
A prokaryote is a simple, single-celled (unicellular) organism that lacks a nucleus and other membrane-bound organelles. The DNA in prokaryotes is found in a central region called the nucleoid. Therefore, based on the information provided in the question, the unknown organism is most likely a prokaryote.